The present invention relates to well cementing design and evaluation methods and more particularly, proposes a combination of formation evaluation, cement design, cement laboratory experiments and cased hole evaluation for better cementing of casings in subterranean wells.
After drilling a well, such as an oil or gas well, the drill pipe is removed and a string of casing is lowered into the wellbore. At this time the drilling mud used to compensate the formation pressure and to remove the formation cuttings from the well is still in the wellbore. In the annulus between the well wall and the casing, this mud needs to be replaced by a cement sheath that holds the casing in place, stabilizes and protects the casing, and to the uppermost point, provides zonal isolation.
Poor zonal isolation results in fluid migrations e.g. water or gas may invade an oil-bearing zone, resulting eventually in a risk of blow out, or to less severe but economically challenging problem such as water production (and the need to provide expensive water treatment surface facilities) or loss of reserves and productions. Remedial work to repair a faulty cementing job is expensive (inasmuch as it increases rig time and delays oil or gas production) and sometime leads to irreparable harm to the hydrocarbon-bearing production.
Evaluating a primary cementing job and eventually electing to perform a remedial treatment is one of the most critical decisions made by the operator during the completion phase of the hydrocarbon wells. Unfortunately, this area is still very ambiguous due to the fact that there is no consistent method or process to address cement evaluation in a systematic manner taking into account the different factors that can affect his primary cement job. Moreover, in response to a demand for cements suitable for deeper, hotter or cooler wells, deviated or horizontal wells, new types of cements and additives have been introduced recently, whose evaluation result in new challenges.
In most cases, poor zonal isolation results from poor mud removal. As mentioned before, the well is initially filled with mud. The cement is placed by pumping a cement slurry downhole trough the casing and back up into the annulus between the casing and the borehole so that the mud is displaced to the surface. In theory, the casing is a perfect cylinder centered inside another perfect cylinder, the well and the cement displaces the mud as in two communicating vases. In the real world, neither the casing nor for the well is cylindrical.
Hydraulic cements set and develop compressive strength as a result of hydration of different cement phases. Although this is a continuous process, three main phases can arbitrarily be defined. In the first phase, the cement slurry has a relatively low viscosity and essentially constant rheological properties. This first phase corresponds to the pumping and placement of the cement downhole. In the second phase, the consistency of the cement increases so that it becomes difficult to pump and place it correctly. However, the developed compressive strength is not enough for the cement to be self-supported and to withstand a significant strength. In the third and last phase, the cement continues to develop compressive strength but the well security is insured and the well construction may be resumed.
During this third phase, the cement is evaluated and a remedial cementing operation may be recommended if the compressive strength is below the expected level. The remedial cementing operation typically consists in isolating the area where the cement evaluation logs revealed low cement compressive strength, perforating the casing and injecting a cement slurry under pressure, a process known as squeeze cementing. Unfortunately, it is often the case that a remedial operation cannot be successfully completed. In many cases the cement cannot be squeezed in the annulus between the casing and the wellbore resulting in fracturing the formation and pumping the cement into the wrong place. Often, this is due to the difficult of interpreting ambiguous results provided by the evaluation tolls.
Moreover, even if this operation is a success, it certainly delays the completion of the well and the beginning of the well production, resulting in significant profit losses for the operator.
It would be suitable to improve the methods of designing and evaluating primary cementing operation to reduce the need for remedial cementing, or improves the efficiency of remedial operations when required.
This invention provides a scientific and systematic method for cement design, job execution and evaluation taking into account wellbore geometry, cement and mud properties, cement job design and execution and evaluation. It also provides a new way to predict cement strength with time.
According to the invention, a significant improvement in cement evaluation is attained by employing a procedure that involves identifying a contaminant, designing a cement slurry, obtaining a set of data related to the development of the compressive strength versus time for said cement slurry at different levels of contamination, pumping the designed cement slurry, evaluating the curing properties of the pumped cement after the cement placement, assessing the degree of contamination of the slurry based on the set of data and predicting the long-term real compressive strength of the contaminated cement.
In one embodiment of the present invention, the step of obtaining a set of data related to the development of the compressive strength versus time for said cement slurry at different levels of contamination includes obtaining correlation curves for each contaminant, showing the linear relationship between contaminated cement compressive strength at different levels of contamination relative to a non-contaminated slurry compressive strength.
Otherwise stated, the cement evaluation log performed after the cementing operation is a xe2x80x98snap shot in timexe2x80x99 used to predict the final quality of the cement rather than the final result on the job. As a result, accurate recommendations can be made on the opportunity of making a remedial cementing.
In most cases, the invention is carried out by a combination of formation evaluation, cement design, cement laboratory experiments and cased hole evaluation for better cementing of casings in particular for oil and gas wells.